This type of touch-sensitive apparatus is known in the art. It may be implemented to operate by transmitting light inside a solid light transmissive panel, which defines two parallel boundary surfaces connected by a peripheral edge surface. Light generated by a plurality of emitters is coupled into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces to a plurality of detectors. The light thereby defines propagation paths across the panel, between pairs of emitters and detectors. The emitters and detectors are arranged such that the propagation paths define a grid on the panel. An object that touches one of the boundary surfaces (“the touch surface”) will attenuate (“frustrate”) the light on one or more propagation paths and cause a change in the light received by one or more of the detectors. The location (coordinates), shape or area of the object may be determined by analyzing the received light at the detectors. This type of apparatus has an ability to detect plural objects in simultaneous contact with the touch surface, known as “multi-touch” in the art.
In one configuration, e.g. disclosed in U.S. Pat. Nos. 3,673,327, 4,254,333 and US2006/0114237, the emitters and detectors are arranged in rows on opposite ends of the panel, and the light is propagated between opposite pairs of emitters and detectors so as to define a rectangular grid of propagation paths.
As an alternative, U.S. Pat. No. 7,432,893 proposes the use of a few large emitters arranged at the corners of the panel, or centrally on each end of the panel, to inject diverging light beams (“fan beams”) into the panel for receipt by arrays of detectors along all ends of the panel. This configuration may enable an increased spatial resolution for a given number of emitters and detectors, by increasing the density of the grid of propagation paths. The spatial resolution indicates the smallest object that can be detected by the touch-sensitive apparatus at a given location on the touch surface.
In an alternative configuration, e.g. disclosed in WO2009/077962, US2011/0234537, US2011/0157096, rows of regularly spaced fan beam emitters and detectors, respectively, are arranged on opposite ends of the panel to define a dense grid of propagation paths across the touch surface.
WO2010/064983 discloses further alternative configurations. In one configuration, which is intended to improve the uniformity of the grid of propagation paths, fan beam emitters and detectors are alternated with equal spacing around the periphery of the touch surface. In another configuration, which is intended to reduce interference phenomena that may occur when different emitters concurrently inject light of the same wavelength into the panel, fan beam emitters and detectors are arranged with randomized spacing around the periphery of the touch surface.
US 2009/0153519 discloses providing alternating emitter and detector ports around the panel. Alternatively, a unique electrode can serve both purposes of emitter and receiver.
In this type of touch-sensitive apparatus, there is a continued desire to improve the spatial resolution with respect to the uniformity of the spatial resolution across the touch surface or the minimum detectable object size at a given position on the touch surface.